function f = grid_min(x,y),

a = [1.078 0.009 -0.285 -0.381 2.573 0.820 1.903 -2.304...
    -1.735 3.625 -0.053 -1.257 -0.107 -0.745 -0.524 -0.720...
    0.723 0.173 -0.199 0.450 -1.151 0.035 0.220 -0.002 0.259...
    0.077 0.076 0.102 0.091 -0.073 0.022 -0.052 -0.057 0.100...
    -0.039 0.124 -0.005 -0.016 0.000 -0.029 -0.002 -0.017 -0.010...
    -0.003 -0.009 -0.004 0.002 -0.002 0.004 0.001 -0.001 0.003...
    -0.006 0.001 -0.005 0.000 0.000 0.000 0.001 -0.000 0.001 0.000...
    0.000 0.000 0.000 0.000]; 
f = polyval2(a,x,y);